Synthesis of carbon nanotubes filled with palladium nanoparticles using arc discharge in solution

ABSTRACT

A novel method for simultaneously forming and filling and decorating carbon nanotubes with palladium nanoparticles is disclosed. Synthesis involves preparing a palladium chloride (PdCl 2 ) solution in a container, having two graphite electrodes, then immersing the graphite electrode assembly, into the PdCl 2  solution; connecting the graphite electrodes to a direct current power supply; bringing the electrodes into contact with each other to strike an arc; separating the electrodes to sustain the arc inside the solution; putting the container with electrode assembly in a water-cooled bath; and collecting Pd-nanoparticles encapsulated in carbon nanotubes and carbon nanotubes decorated with Pd-nanoparticles. The temperature at the site of the arc-discharge is greater than 3000° C. At these temperatures, the palladium is ionized into nanoparticles and the graphite electrodes generate layers of graphene (carbon), which roll away from the anode and encapsulate or entrap the Pd-nanoparticles. The unique nanotube structures have significant commercial potential as gas sensors or as a means for hydrogen storage.

This invention claims the benefit of priority to U.S. Provisional PatentApplication Ser. No. 60/487,711 filed Jul. 16, 2003.

FIELD OF THE INVENTION

This invention relates to the synthesis of carbon nanotubes containingmetals, in particular to novel carbon nanotubes filled with palladiumnanoparticles and a method of manufacture.

BACKGROUND AND PRIOR ART

Carbon nanotubes are strong tubular structures formed from a single ormulti-layer of carbon atoms measured in billionths of a meter(nanometer) in diameter. Carbon nanotubes are proclaimed to be strongerthan diamonds and more expensive than gold with significanttechnological potential. Potential applications can include flat paneldisplay in telecommunications devices, fuel cells, lithium-ionbatteries, high-strength composites, novel molecular electronics, gassensors, and a means for hydrogen storage.

Recent developments include filling the hollow cavity of the tiny,thread-like carbon nanotubes to control or influence nanotube behaviorand functionality. The bulk of nanotube production is still a challengebecause it is very expensive—more than gold.

Undeterred by costs, researchers have developed several methods forfilling nanotubes with metal oxides, pure metals and other materials.The nature of the filling is dependent on the method used to introducethe materials to the nanotube cavity with some methods giving discretecrystalline filling and molten media giving long, continuous crystals.One disadvantage of prior art methods of filling nanotubes is that thecrystals and the long continuous fibers have a limited surface area,thus limiting the functional capacity for various applications.

Carbon nanotubes (CNTs) are usually filled using post-processing stepswhich involve opening up and filling through either capillary action orother chemical means. Such additional filling steps are not onlyinefficient, but also additive to the overall production cost. Thus, thesearch for new, interesting, affordable filled-carbon nanotubescontinues.

The synthesis of metal-filled carbon nanotubes has tremendous potentialfor technological applications, such as, in gas sensing, catalystsupports and hydrogen storage wherein large surface areas are required.Thus, the palladium nanoparticle-filled carbon nanotubes and method ofmanufacture of the present invention have significant commercialpotential.

SUMMARY OF THE INVENTION

The first objective of the present invention is to provide aninexpensive, one-step method for making filled-nanotubes thatsimultaneously fills and decorates the nanotubes during its synthesis.

The second objective of the present invention is to provide a simplifiedarc-discharge in solution method for the synthesis of carbon nanotubesfilled with palladium nanoparticles.

The third objective of the present invention is to produce palladiumnanoparticles with a diameter of approximately 3 nanometers (nm) insidecarbon nanotubes.

The fourth objective of the present invention is to produce carbonnanotubes with diameters of approximately 15 nm in each nanotube.

The fifth objective of the present invention is to simultaneously formand encapsulate palladium nanoparticles in the hollow cavity of carbonnanotubes.

The sixth objective of the present invention is to provide carbonnanotubes filled and decorated with palladium nanoparticles that have anenhanced ability for hydrogen storage.

The sixth objective of this invention is to provide carbon nanotubesfilled and decorated with palladium nanoparticles that function as gassensors.

The seventh objective of this invention is to provide carbon nanotubesfilled and decorated with other metal nanoparticles and their compoundsconsisting of oxides, sulfides, carbides, nitrides, halides, and thelike, for various other applications.

Filling the carbon nanotubes with metallic nanoparticles, especiallypalladium, is to enhance the hydrogen storage ability of carbonnanotubes (CNTs). The CNTs filled and decorated with palladiumnanoparticles have been characterized using high-resolution transmissionelectron microscopy (HRTEM) equipped with energy dispersive spectroscopy(EDS) system, transmission electron microscopy (TEM), scanning electronmicroscopy (SEM) and X-ray photoelectron spectroscopy (XPS) for size,morphology, chemical constituent and chemical state.

Further objects and advantages of this invention will be apparent fromthe following detailed description of a presently preferred embodimentthat is illustrated schematically in the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A shows a scanning transmission electron microscopy (STEM) imageof a large number of Pd-filled carbon nanotubes.

FIG. 1B shows the energy dispersive spectroscopy (EDS) spectrumconfirming the presence of carbon and palladium in the nanotubes.

FIG. 2A shows a high-resolution transmission electron microscopy (HRTEM)image of a CNT filled with palladium (Pd) nanoparticles.

FIG. 2B shows lattice fringes of Pd nanoparticles in the size ofapproximately 3 nm under high resolution.

FIG. 2C shows a selected area diffraction pattern (SAED) pattern ofPd-nanoparticles inside the CNT showing various lattice planes.

FIG. 3 is a schematic diagram showing the formation of the CNT filledwith Pd-nanoparticles in the arc-discharge in solution method.

FIG. 4 is a diagram of a complete experimental set-up for makingnanoparticle filled CNTs using arc-discharge in solution.

FIG. 5 is a schematic diagram of a reaction cell for arc-discharge insolution.

FIG. 6 a is a graph of the weight change of cathode and anode electrodeswith time during arc-discharge in solution.

FIG. 6 b is a graph of the combined weight change of anode and cathodeelectrodes with time during arc-discharge in solution.

FIG. 7 a is a scanning electron micrograph (SEM) of a graphite rodbefore the in-situ synthesis of palladium filled and decorated carbonnanotubes (CNTs).

FIG. 7 b is a scanning electron micrograph (SEM) of the anode, tenminutes after the arc-discharge.

FIG. 7 c is a scanning electron micrograph (SEM) of the cathode, tenminutes after the arc-discharge.

FIG. 7 d shows images of graphite rods before the in-situ synthesis ofpalladium filled and decorated CNTs.

FIG. 7 e shows images of graphite rods after the in-situ synthesis ofpalladium filled and decorated CNTs.

FIG. 8 is a scanning electron micrograph (SEM) of CNTs taken from thetip of the cathode.

FIG. 9 is the high-resolution transmission electron microscopy (HRTEM)image of a CNT showing the inner and outer diameter.

FIG. 9 a is an enlarged inset micrograph that reveals the distancebetween two concentric walls.

FIG. 10 is a transmission electron microscopy (TEM) bright-field imageshowing marked black spots that are palladium nanoparticles decoratedaround a CNT.

FIG. 11 is an energy dispersive spectroscopy (EDS) spectrum of CNTdecorated with palladium collected from transmission electron microscopy(TEM) data.

FIG. 12 a is a zero-loss energy filtered transmission electronmicroscopy (TEM) image.

FIG. 12 b is a transmission electron microscopy (TEM) energy-loss map ofpalladium.

FIG. 12 c is a transmission electron microscopy (TEM) energy-loss map ofcarbon.

FIG. 13 a is a high-resolution transmission electron microscopy (HRTEM)image of a CNT with palladium nanoparticles shown as black spots.

FIG. 13 b is a scanning transmission electron microscopy (STEM) image ofthe same nanotubes in 13 a wherein the palladium nanoparticles decoratedaround the nanotubes are shown as white spots.

FIG. 13 c is a high-resolution transmission electron microscopy (HRTEM)image showing the lattice fringes of the CNT.

FIG. 14 is a Fourier filtered diffraction pattern of palladiumnanoparticles on CNT.

FIG. 15 is a deconvoluted X-ray photoelectron spectroscopy (XPS)envelope of the CNTs decorated with palladium nanoparticles.

FIG. 16 is a transmission electron micrograph (TEM) of dislodgedgraphene sheets showing the wavy morphology.

FIG. 17 a is a transmission electron micrograph (TEM) revealing innerwalls capped in a periodic manner and the same number of walls for bothsides of a multi-walled CNT.

FIG. 17 b is a transmission electron micrograph (TEM) revealing anunequal number of walls in two sides of a multi-walled CNT.

FIG. 17 c is a transmission electron micrograph (TEM) revealing an equalnumber of walls for both sides of a multi-walled CNT.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Before explaining the disclosed embodiment of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangement shown since theinvention is capable of other embodiments. Also, the terminology usedherein is for the purpose of description and not of limitation.

Carbon nanotubes (CNTs) are classified into two types; a singlehexagonal mesh tube called a single-walled carbon nanotube abbreviatedas (“SWCNT”) and another comprising a tube of a plurality of layers ofhexagonal meshes called a multiwalled carbon nanotube (abbreviated as“MWCNT”). The synthesis and product of the present invention isappropriate for both SWCNT and MWCNT.

Reference is made to “encapsulating” and “decorating” CNTs; both wordsare used to describe the filling of a CNT with nano-sized particles.“Encapsulating” provides a minimum number of nano-sized particles to theinterior cavity of a CNT. “Decorating” is used to mean impregnating theinner and outer walls of the CNT and thereby incorporating a maximumnumber of nano-sized particles in a CNT. The impregnating of the innerand outer walls of the CNT occurs during the simultaneous formation ofthe CNT and the entrapment of the nano-sized particles during the arcdischarge in solution.

FIG. 1A shows a scanning transmission electron microscopy (STEM) imageof the SWCNT. The STEM micrograph reveals the presence of a largenumbers of Pd-filled, carbon nanotubes, and one such nanotube isindicated by an arrow 10 in FIG. 1A.

FIG. 1B is an energy dispersive spectroscopy (EDS) spectrum thatconfirms the presence of carbon 11 and palladium (Pd) 12 in thenanotubes. A copper (Cu) grid is used in the TEM study, hence a Cu peak13 was also present in the EDS spectrum. The EDS spectrum did not revealthe presence of chlorine, which indicates that the palladium ions in thesolution were reduced to metallic palladium during the arc discharge insolution synthesis.

An HRTEM image 20 of a CNT filled with Pd-nanoparticles is depicted inFIG. 2A. The carbon nanotube shown in FIG. 2A has a diameter of about 15nm. FIG. 2A also indicates the presence of dark regions inside thenanotube. Further investigation with the high-resolution mode on thoseregions shows the lattice fringes 21 of palladium nanoparticles in thesize of about 3 nm, as presented in FIG. 2B. Thus, FIG. 2B furtherconfirms that the presence of a Pd peak in the EDS spectrum in FIG. 1Bwas due to these nanoparticles inside the CNT. In order to investigatethe crystal structure of these encapsulated Pd-particles, selected areadiffraction pattern 23 (SAED) was collected in the TEM and is depictedin FIG. 2C. The intensity profile 24 of the SAED pattern is presented asan inset at the bottom right corner of FIG. 2C, which clearly indicatesthe presence of nine different diffraction peaks corresponding to ninediffraction rings in the SAED. After indexing the diffraction pattern,it is confirmed that the crystal structure of these Pd-nanoparticles areface-centered cubic. SAED pattern 23 of Pd-nanoparticles inside the CNTshows various lattice planes [a: 11; b: 200; c: 220; d: 311; e: 222; f:400; g: 331; h: 420; i: 422].

Although the exact mechanism of the reduction of PdCl₂ intonanoparticles of Pd metal is not clear; the following possibilities areplausible explanations. Reduction of palladium ions into atomicpalladium in the solution can take place with the help of reducinggases, such as, carbon monoxide and hydrogen, which are formed duringthe arc-discharge process in the solution. The temperature at the siteof the arc is greater than 3000° C. Hence, the plasma region produced bythe arc adjacent to the electrodes is enveloped by a solution-vaporinterface. There is a substantial thermal gradient across the plasmaregion. The decomposition temperature of palladium chloride is 738° C.at ambient atmospheric pressure. Therefore, the palladium chloride ispossibly thermally decomposed to atomic Pd and chlorine gas near theelectrodes. Subsequently, chlorine atoms combined to form chlorine gas,which escaped with water vapor and carbon monoxide, which is the reasonfor not detecting chlorine in the EDS spectrum.

Another explanation is that palladium ions (Pd²⁺) are reduced to Pdatoms taking electrons from the plasma formed due to the arc discharge,an excellent source of electrons. Palladium nanoparticles having adiameter of 3 nm are formed after the arc discharge in PdCl₂ solution.

The formation of CNTs in the present invention is accomplished by aknown method. Graphite anode consists of two-dimensional hexagonalarrays of carbon atoms which are known as graphene. Such graphene orgraphitic carbon sheets are rolled from the anode, thereby forminggraphitic carbon tubules, at high temperatures, during the arc dischargein the solution. Therefore, palladium nanoparticles 30 simultaneouslyformed, are trapped and encapsulated in the carbon nanotubes duringrolling of graphitic layer 31 from the graphite anode. A schematicdiagram of simultaneous formation and encapsulation of Pd nanoparticlesin CNTs 32 is shown in FIG. 3.

FIG. 4 is a diagram of a complete experimental set up for makingnanoparticle-filled carbon nanotubes. The reaction chamber or container40 is fitted with two graphite electrodes 41, 42. Electrode 41 functionsas a cathode and electrode 42 functions as an anode. A direct currentpower source 43 is connected to the anode and cathode to supply powerfor the arc-discharge. Conveniently positioned near the reaction chamber40 is the chiller 44, which contains a water-cooled bath having atemperature of approximately 7° C. The set-up in FIG. 4 can be used tosynthesize a wide variety of carbon nanotubes filled with metalnanoparticles. The carbon nanotubes can be single-walled ormulti-walled. Below is an example of the preparation of palladium-filledcarbon nanotubes; however, it should be understood that a wide varietyof metallic-filled carbon nanotubes can be produced using the processdisclosed herein.

EXAMPLE

A palladium chloride solution of 2 milli molar (mM) concentration isprepared by dissolving PdCl₂ powder with 99.999% purity in de-ionizedwater. In order to ensure complete dissolution of the palladiumchloride, hydrochloric acid is added to have its concentration 0.1 molar(M) in the solution. Palladium chloride and hydrochloric acid areobtained from Sigma-Aldrich Chemical Company. The resistance of thede-ionized water used in the present invention is greater than 10 MΩ.Synthesis of CNTs filled with Pd-nanoparticles is carried out in areactor containing the palladium chloride solution. Two graphiteelectrodes, acting as a cathode and an anode, are immersed in thereactor containing the 2 mM PdCl₂ solution. The diameters of thegraphite rods used for cathode and anode are 3.05 mm and 12.70 mm,respectively. The anode and cathode materials are obtained from AlfaAesar with 99.9995% and 99.0% purity, respectively. A direct currentpower supply (Model: DUAL MIG 131/2) manufactured by Chicago Electric,is connected to the graphite electrodes, which are immersed in thepalladium chloride solution. The electrodes are brought in contact witheach other to strike an arc and then are separated immediately to adistance of approximately 1 mm in order to sustain the arc inside thesolution for a certain period of time. Graphene sheets from the anodeare detached and roll away at high temperatures during arc-discharge insolution. The reactor with electrode assembly is put inside awater-cooled bath, having a temperature of approximately 7° C., in orderto avoid the excessive heating of the PdCl₂ solution. Palladiumnanoparticles encapsulated within or attached to carbon nanotubes aresynthesized from palladium chloride solution through the arc dischargemethod at an open circuit potential of 28 V with an optimized directcurrent of 35 amperes (A). Palladium metal filled carbon nanotubes arecollected and studied further using HRTEM (Model: Philips 300 TECNAI) at300 kV for their size and the crystal structure.

Another especially designed reactor cell shown in FIG. 5 was used forin-situ, one-step synthesis of CNTs decorated or filled with palladium(Pd) nanoparticles. The reactor cell has four main components; a cellcavity 50 a direct current (DC) power supply system (not shown), afiltering unit and a chilling loop. In FIG. 5, the schematic diagramshows a cell with a height of approximately 20 centimeters (cm) thatconsists of double walled glass 52, 53 with 2 cm thickness for flowingof cold water at 7° C. to facilitate cooling. The inner diameter of thecylindrical cell is 10 cm. The cell has two inlets 54, 56 and twooutlets 58, 60. The inlet 54 is paired with outlet 60 that are bothconnected to the volume between the double glass walls 52, 53 for theflow of chilled water used to cool the cell. The other inlet 56 ispaired with outlet 58 which are both connected directly to the interiorof the cell. Inlet 56 is used for filling the cell with the solution forthe arc-discharge reaction and filtering out through outlet 58, theencapsulated and decorated nanotubes from the cell. Two stainless steelplates 62, 64 are used to connect the anode and cathode electrodes.

Extensive analysis of the arc-discharge reaction, electrodes andreaction products are described in detail below.

FIG. 6 a represents the weight change of each electrode at two-minuteintervals. There was less than 0.5 gram weight increase in the weight ofthe cathode and up to 2.0 gram weight decrease in the weight of theanode during a ten minute interval. FIG. 6 b represents the cumulativeweight change of both the cathode and anode at two-minute intervals overa period of ten minutes. The graph clearly documents a decrease incumulative weight of the electrodes. The cathode gains weight with time,whereas the anode loses weight with time during the arc discharge. Theoverall weight of the electrodes decreases with time.

A portion of the excessive heat produced in the solution during arcingcauses evaporation of the solution. The evaporation rate of the waterduring arc-discharge in solution in the present invention was found tobe 3.2 cm³·min⁻¹. The temperature at the site of the region of the arcis expected to be greater than 3500° C. Hence, the plasma regionproduced by the arc adjacent to the electrodes is enveloped by asolution-vapor interface. There is a substantial thermal gradient acrossthe plasma region. The temperature at the anode edge is more than 3000°C., while it is 100° C. at the solution-vapor interface. The loss ofweight of the electrodes is due to the formation of carbon dioxide,carbon monoxide, carbon nanotubes (CNTs) and other carbonaceousmaterials including dislodged graphene sheets, carbon onions, amorphouscarbon and carbon rods.

FIG. 7 a shows the surface of an electrode prior to the arc dischargereaction. FIG. 7 b shows the anode and FIG. 7 c shows the cathode afterarc discharge in water, respectively. The structure of the anode surfacehas significantly changed after the arc discharge. The morphology of thecathode surface is also uneven due to the deposition of the carbonaceousmaterials. FIG. 7 d is an overall picture of the cathode and anodebefore arc discharge and FIG. 7 e is an overall picture of bothelectrodes after arc discharge; the cathode increased in size, the anodedecreased in size.

Scanning electron micrograph (SEM) studies of the cathode materialsreveals a deposition of CNTs. FIG. 8 shows a small mound of depositedCNTs. The inner and outer diameters of a CNT as shown in HRTEMmicrograph of FIG. 9 are 3 nm and 10 nm, respectively. The insetpicture, FIG. 9 a, shows that the distance between the two concentricwalls is 0.359 nm.

A transmission electron micrograph (TEM) bright-field image of a CNTdecorated with palladium nanoparticles is shown in FIG. 10. The diameterof the CNT is approximately 15 nm. The spherical darker regions 101,102, 103 in FIG. 10 correspond to the palladium nanoparticles of about 3nm in diameter.

The EDS spectrum in FIG. 11 is very similar to that in FIG. 1B andreveals the presence of carbon (C) and palladium (Pd). A copper (Cu)grid was used in the TEM study; hence a Cu peak is also observed in thespectrum. Again it is observed that the EDS spectrum does not show achlorine peak which suggests that chlorine was not present either aspalladium chloride inside the nanotubes or as atomic chlorine attachedto the sidewall.

A zero-loss energy filtered image of a CNT decorated with palladiumnanoparticles is shown in FIG. 12 a. A dense agglomeration of palladiumnanoparticles 120 in the center of the CNT can be observed. To confirmfurther, a carbon map and a palladium map have been collected using theGatan imaging filter on the same portion of the CNT. The palladium mapof the CNT is shown in FIG. 12 b and the carbon map of the CNT is shownin FIG. 12 c. FIG. 12 b shows that the bulging area of the CNT is causedby the deposition of several palladium nanoparticles. The palladium mapalso reveals that the CNT is decorated with palladium nanoparticles,however, FIG. 12 b does not clearly show that all nanoparticles areoutside the CNT. HRTEM is used to investigate the actual position of thepalladium nanoparticles.

A CNT that is decorated by well-separated palladium nanoparticles isshown in FIG. 13 a. HRTEM micrograph shows the lattice fringes of boththe CNT and the palladium nanoparticles. FIG. 13 b shows the same CNT asin FIG. 13 a investigated with a scanning transmission electronmicroscopy (STEM) using a high-angle annular dark field detector forZ-contrast imaging. Because of the high atomic weight, palladiumnanoparticles in FIG. 13 b appear as bright spots. HRTEM micrograph inFIG. 13 c shows the lattice fringes of the CNT.

In order to investigate the crystal structure of the palladiumnanoparticles, selected-area diffraction patterns (SAED) were collectedin the TEM mode. FIG. 14 shows a Fourier-filtered SAED pattern with ninedistinct Debye-Scherrer rings compatible with a face-centered cubiccrystal structure of the palladium particles. The de-convoluted X-rayphotoelectron spectroscopy (XPS) spectrum with the Pd(3d) envelope isshown in FIG. 15 revealing the presence of Pd(3d_(5/2)) and Pd(3d_(3/2))peaks at binding energy values of 335.6 and 340.9 eV, respectively. Thepresence of a small amount of palladium oxide has also been observed inthe XPS envelope as shown in FIG. 15.

The carbon nanotubes (CNTs) formed during the arc discharge in solutionprocess were studied using transmission electron microscopy (TEM). Itwas observed that the original graphene sheets were partially rolled upleaving behind some bath tub-shaped portions, suggesting that rolling ofgraphitic layers from the anode materials formed the CNTs. In FIG. 16the HRTEM micrograph shows a dislodged graphene sheet with a wavysurface morphology, which supports the scroll mechanism for theformation of CNTs.

The formation of multi-walled CNTs during the arc discharge in solutionprocess is shown in FIGS. 17 a, 17 b, and 17 c. FIG. 17 a shows a HRTEMmicrograph of a CNT with nested cylindrical graphitic layers. FIG. 17 bshows different numbers of walls on the two sides of a CNT with a finalclosure by a single cap. FIG. 17 b also reveals the presence ofincomplete and bent layer at the inner concentric wall 170, as marked byarrows. Such defects could initiate the formation of an inner cap withtime. The scroll mechanism can form a convoluted multi-walled CNT, whichcould eventually transform to a concentric multi-walled CNT by therearrangement of carbon atoms. FIG. 17 c shows the same numbers of wallsin the two sides of a CNT, which may further support the scrollmechanism. Thus, the in-situ synthesis of Pd-nanoparticles decorated andencapsulated CNTs using arc-discharge in solution is applicable to bothsingle-walled and multi-walled CNTs.

The present invention successfully synthesizes carbon nanotubes filledwith metallic palladium nanoparticles by using a method of arc dischargein a solution containing palladium chloride. The diameters of nanotubesand the Pd-nanoparticles are measured as 15 nm and 3 nm, respectively.Such CNTs filled with palladium nanoparticles are “vessels” withenhanced capability for hydrogen storage.

There are many advantages to the process for simultaneously forming CNTswith Pd-nanoparticles, including, but not limited to, the simplicity ofthe entire process, the efficiency of the filling process, and theadvancement of the entire field of nanotechnology.

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

1-13. (canceled)
 14. A carbon nanotube filled with Pd-nanoparticles madeby the process of: preparing a palladium chloride (PdCl₂) solutioncomprising PdCl₂ powder, de-ionized water, and hydrochloric acid in acontainer; immersing a graphite electrode assembly, having two graphiteelectrodes, wherein the electrode assembly consists of one electrodeacting as a cathode and another electrode acting as an anode, into thePdCl₂ solution; connecting the graphite electrodes to a direct currentpower supply; bringing the electrodes into contact with each other tostrike an arc and create an arc-discharge, wherein the temperature atthe site of the arc-discharge is greater than 3000° C.; vaporizing thePdCl₂ solution to leave a residue of Pd-nanoparticles; separating theelectrodes to sustain the arc inside the solution; forming graphenelayers that roll away from the electrodes and simultaneously encapsulatethe Pd-nanoparticles; putting the container with electrode assembly in awater-cooled bath that has a temperature of approximately 7° C.; andcollecting Pd-nanoparticles entrapped in carbon nanotubes.
 15. Acomposition of matter comprising simultaneously formed carbon nanotubesfilled with metallic-nanoparticles prepared by a process of in-situ arcdischarge in solution using a cylindrical reactor cell comprising:double walled glass with 2 cm thickness, on two sides of a cylindricalcell having an inner diameter of 10 cm, for flowing of cold water at 7°C. to cool the cell; a cell cavity with a pair of inlets and a pair ofoutlets, wherein a first inlet is paired with a first outlet connectedto the volume between the double glass walls for the flow of chilledwater, and a second inlet is used for filling the cell with a solutionfor the arc-discharge reaction and is paired with a second outlet forfiltering out the encapsulated and decorated carbon nanotubes formed inthe cell; a first stainless steel plate covering the top of thecylindrical reactor cell that connects to an anode and a secondstainless steel plate covering the bottom of the cylindrical reactorcell that connects to a cathode; and a direct current power supplysystem connecting to both the anode and the cathode that are immersed inthe solution for the arc-discharge reaction.
 16. The composition ofmatter in claim 15, wherein the solution for the arc discharge reactionincludes a solution selected from at least one of metallic chlorides andmetallic oxides for synthesizing carbon nanotubes filled with metallicnanoparticles.
 17. (canceled)
 18. The composition of matter in claim 16,wherein the solution for the arc discharge reaction includes palladiumchloride for synthesizing carbon nanotubes filled with palladiumnanoparticles.